The invention relates to a displacement device provided with a first part comprising at least two contact bodies which can be pressed against a second part and are movable stepwise relative to the first part by means of an actuator such that the first and the second part are displaceable relative to one another in a displacement direction under the influence of a friction force present between the contact bodies and the second part.
The invention also relates to a displacement unit suitable for use in a displacement device according to the invention.
A displacement device of the kind mentioned in the opening paragraph is known from EP-A-0 592 030. The first part in this known displacement device comprises a holder, and the second part comprises a shaft which is displaceable relative to the holder in an axial direction. The contact bodies of the first part each comprise a clamping member which can be clamped against the shaft. The first part further comprises a transport member which is fastened to the holder and by means of which a distance present in axial direction between the clamping members can be changed. The shaft is displaceable stepwise in the axial direction in accordance with a displacement cycle by means of the clamping members and the transport member. A first one of the two clamping members is clamped against the shaft at the start of a displacement cycle. Then the distance between the clamping members is increased by means of the transport member, so that the shaft is displaced in axial direction by the first clamping member. Subsequently, the second clamping member is clamped against the shaft, and the first clamping member is deactivated. The distance between the clamping members is reduced again now by means of the transport member, so that the shaft is displaced farther in axial direction by the second clamping member. Finally, the first clamping member is clamped against the shaft again, while the second clamping member is deactivated, whereupon the displacement cycle is repeated. The clamping members and the transport member are manufactured from piezoelectric material, so that very accurate displacements of the shaft relative to the holder in the axial direction are achieved.
A disadvantage of the known displacement device is that a position in which the shaft is present relative to the holder, as seen perpendicular to the displacement direction, is not accurately defined. The shaft is supported by the clamping members perpendicularly to the displacement direction. The positions of the clamping members relative to the holder perpendicular to the displacement direction, however, are dependent on inter alia the diameter of the shaft and are influenced by deviations of the shaft diameter in the displacement direction and by local deviations in the shaft surface. The known displacement device thus does not have an accurate bearing support function, with the shaft occupying an accurately defined position in a direction perpendicular to the displacement direction. This disadvantage renders the known displacement device unsuitable for a large number of applications.